rim sizes

[dal1]

trying to build my first new school bike  with a few school bits thrown in and i have just bought some wheels off e bay .when they have arrived they look very small to me 21 mm in width .is this what i would have called skinnys bitd.i think i have been done? if i bought some more rims could i have them rebuilt with the same hubs and spokes?

[MartyC]

Sounds like you have bought either 20 x 1 3/8's, 20 x 1 1/8's or 20 x 1.5's.  What is the overall diameter of the rims and what make are they?  1 1/8's and 1 3/8's will have the same outside diameter.